Electromagnetic trackers are sensitive to conductive or ferromagnetic objects. Presence of metallic targets near to an electromagnetic transmitter (Tx) or receiver (Rx) may distort the transmitting signals resulting inaccurate position and orientation (P&O) measurement.
Conventionally, there are varying approaches for recovering the distorted signals (or sensor P&O) for applications such that the sensor position is fixed relative to the surrounding metals. Indeed, the following reference shows many of the conventional approaches: V. V. Kindratenko, “A survey of electromagnetic position tracker calibration techniques”, Virtual Reality: Research, Development, and Applications, pp. 169-182, Vol. 5, No. 3, 2000. The most popular approach is called “P&O mapping.” With P&O mapping, the distortion is corrected by function-fitting the distorted P&O measurements to the undistorted counterparts based on a look-up table built on a desired tracking space.
The conventional approach of P&O mapping has proven to be a reliable solution for many applications, including fluoro-tracking applications. However, under the circumstance that the signals are significantly distorted (E.g., by conductive shield next to the sensor), the electromagnetic tracker may fail in processing the distorted signals and thus produce invalid P&O for distortion mapping. This largely degrades tracker performance and reliability.
Failure of the tracker in dealing with significant field distortion is primarily due to instability of conventional tracking algorithms. In conventional approaches, the directly-calculated P&O is optimized by using a fitting algorithm to best-fit the signal measurements to an analytical dipole model. Conventionally, the fitter tends to be very unstable in handling the measurements which are considerably deviated form the model predictions.